Pigments are inorganic or organic, chromatic or non-chromatic coloring materials which are practically insoluble in the medium which they are employed in. Many inorganic pigments also function as fillers or vice versa. Pigments are more especially employed for coloring lacquers and paints, for coloring plastics, paper, textiles, cement, concrete, ceramics, glass, enamels, cosmetics, foodstuffs and furthermore for inks in the printing industry and as artists' coloring materials.
It is more particularly in the paper making industry that products are demanded which possess a high gloss. The German patent 2,733,722 relates to a method for the continuous production of calcium sulfo-aluminate pigment with a particle size of 0.1 to 2.0 .mu.m by the reaction of calcium hydroxide with aluminum sulfate for paper coating purposes. As applied to paper for printing purposes and the like this method leads to a high gloss even in the case of only moderate calendering conditions owing to the high degree of hydration of sateen white, paper with a high opacity being obtained by the formation of a thick coating layer.
The German patent 1,938,162 relates to a method for the coating of paper or the like, wherein a coating material in the form of a polymeric composition comprised in a liquid medium is applied to the web. The use of this method is to render possible the direct production of a light coating with a high degree of smoothness and opacity without subsequent smoothing operations being necessary.
The German patent 3,617,169, however, discloses that when employed as the sole pigments carbonate-containing pigments, more particularly CaCO.sub.3 pigments, produce substantial technical advantages. The present invention accordingly relates to a coating color which as pigments exclusively contains carbonate-containing pigments.
It is furthermore known that carbonate-containing pigments which have a high specific surface area may be advantageously employed as pigments for high gloss paper. Thus the German patent 4,400,566 discloses a method for the production of natural and/or synthetic carbonates, more especially calcium carbonates, which possess a specific surface area of over 20 m.sup.2 /g as determined by the BET/N.sub.2 method and preferably 20 to 50 m.sup.2 /g using the same method. Such carbonate-containing pigments are produced by wet grinding milling with recirculation, that is to say with feedback of the product leaving the mill so many times that the final product is produced with the desired specific surface area.
It has been known for a long time that the gloss of a paper is dependent on the grain size distribution of the pigment contained therein.
Pigments must generally be milled in order to render them suitable for the various different fields of application. This will now be explained in the following example in detail with reference to a paper coating composition. Coated papers are coated with a composition essentially containing an adhesive or binding agent and a pigment in the form of a mineral filler. For a description of the components of paper coating materials and the application thereof, see the book by James P. Casey, "Pulp and Paper Chemistry and Technology," chapter XIX, vol. III (1961). The binding agent employed may consist of for example starch, casein or a synthetic latex. The specific binding agent employed will for example be dependent on the printing process used; thus offset litho printing requires a binding agent which is insoluble in water.
One group of calcium carbonate pigments consists a naturally occurring material. Such a material includes limestone, marble and chalk.
The normally commercially available natural prepared chalk pigments incline to form low-gloss paper coatings. This is to be attributed to the fact that none of them contain more than 35% by weight of particles under 2 microns.
Taking this as a starting point the German patent publication 1,696,190 B proposes a paper coating composition for glossy coated paper, which is characterized in that natural chalk is worked to such an extent by milling with sand or in a jet mill or by particle size-based classification that it contains at least 60% by weight of particles under an equivalent spherical diameter of 2 microns, and does not contain more than 5% by weight of particles larger than an equivalent spherical diameter of 10 microns, and not more than 0.05% by weight of particles which are larger than 53 microns in size.
Speaking quite generally it is to be observed that the particle size has an substantial influence on the properties of the product containing the filler, as for example on the gloss of a coated paper. The German patent 2,808,425 lists 9 literature references from which it appears that gloss increases with an increase in the proportion in the pigment of the finest particles. This means that at the time it was assumed that as regards gloss there was no optimum level for the content of finest particles of the pigment, for the opinion was that the finer the particles of the pigment, the higher the gloss.
The teaching of the said German patent 2,808,425 was the recognition that this previous teaching was superseded, for in accordance with the said German patent 2,808,425 a still higher gloss could be obtained if ultra-fine particles, i.e. particles &lt;0.2 .mu.m, were completely or substantially avoided. This meant that there is in fact an optimum as regards gloss, namely as regards the value for particles &lt;0.2 .mu.m.
Finally in connection with the prior art it is to be observed that the gloss of coated paper increases with each increase in the solids content of the coating color.